Stavudine (d4T) and didanosine (ddI) are commonly used as single agents for treatment of HIV infection. The FDA has approved use of ddI as initial therapy for HIV infected adults and children. ACTG 152 recently demonstrated the superiority of ddI over ZDV monotherapy and the equivalent effect of DDI + ZDV as compared with ddI alone. Accumulating information suggests that combination therapy with two or more antiretroviral agents, however, may hold more promise than single-agent therapy in the long-term management of HIV infection. Pilot trials of ddI + d4T in HIV infected children with advanced HIV disease indicated improvement in virologic and immunologic status, with no significant increase in adverse events. ACTG 327 is a three armed study of approximately 143 HIV infected children ages six months to ten years who have received either ACTG 240 study treatment or prescription ZDV for six months or more. The 48 week, randomized, double-blind, placebo- controlled study will evaluate the safety of combination therapy as well as pharmacokinetics of the study medications and the effect on plasma HIV RNA concentrations.